As miners look to reclaim more minerals from the flotation process through froth pumping they are potentially exacerbating existing problems in their circuit design, according to Les Harvey, Regional Product Manager for Slurry Pumps at Weir Minerals.
Mining companies are making these moves to counteract declining ore grades, but, occasionally these techniques are deployed without making allowances for the design of the mine’s froth pumping equipment.
Froth pumping remains one of the most complex engineering challenges in minerals processing, as air management issues in the hopper, sump and pump itself lead to inefficient pumping, increased maintenance and even lost product, according to Weir.
“We’ve started to notice a pattern among our customers who are having trouble with their froth pumps,” says Harvey. “By using more flocculants and other chemicals designed to improve mineral recovery, they’re exacerbating existing problems in circuit design and reducing the returns they’re looking for.”
Close examination of the froth’s makeup and physical qualities is often needed to resolve issues. Ensuring operators’ froth handling equipment adheres to best design practices is an important first step in resolving problems, according to Weir.
Maintaining pressure in the pump
The key challenge in froth pumping is dealing with air in the pump itself, as it tends to naturally centrifuge into the impeller’s eye where it builds up into an ‘air lock’ which impedes the movement of slurry through the pump, Weir said.
In addition to reducing the pump’s efficiency, the air build up in the pump will reduce the flow through the pump and increase the slurry level in the suction hopper. The increased slurry level may push the pocket of air through the pump, causing surging and excessive vibration which can damage the pump bearings, impeller and shaft.
“The best way to manage air in a froth pump is to invest in a froth pump with a Continuous Air Removal System (CARS), which we have in our Warman AHF, MF and LF pumps,” Harvey says.
CARS allows air to move from the pump’s impeller eye to an air collection chamber in the back through a vent hole in the impeller. From that chamber, a flow inducer removes the air from the pump through a vent pipe.
Harvey said: “It’s also important to position the pump’s discharge pipe at the top of the pump, or at a 45° angle as this will give air trapped at the top of the casing a way to escape the pump.”
Solving problems in the sump and hopper
A persistent problem Weir sees is when hoppers designed to meet the demands of slurry pumping are used in a froth pumping application, Harvey said. “Slurry hoppers require turbulence to prevent the mineral content from settling, while turbulence in a froth pump prevents the air from escaping and leads to blockages.”
Tanks designed for froth pumping promote continuous circular movement, where solids and liquids are sent to the outside of the sump for further transport while air centrifuges into the centre where it can be removed. This ‘whirlpool’ movement can be encouraged by introducing the slurry from the top of the tank at a tangential angle, according to Weir.
Conical designs, rather than those with a flat or rounded floor, further improve the flow of minerals and froth into the pump, the company added.
Smooth sailing from the tank to the pump
To prevent blockages, the intake pipe which links the tank to the pump should be large diameter and slope downwards towards the pump, according to Weir. This design allows escaped air to separate and travel back up the pipe where it can escape from the sump, rather than build up into blockages.
Harvey said: “The shorter your intake pipe, the harder it is for blockages to build up. However, in addition to a maintenance spool and isolation valve, it’s a good idea to leave enough space for a water injection port, which is useful for flushing out any solids build up.
“To make maintenance easier, a dump valve can be included on the suction side of the pump, between the pump and the isolation valve. This will allow you to drain slurry from the pump and discharge pipe system when stopping the pump for maintenance.”
Understanding tenacious froths
Froths are often classified as either brittle – with large air bubbles that break easily – or tenacious – where air forms tight bubbles around minerals and is difficult to separate. Froth being more tenacious than was accounted for is a frequent cause of blockages as air cannot effectively be removed, Weir says.
Harvey said two things are happening in the market today: One, mine operators are grinding the product much finer than before to liberate more from the waste rock. Two, they’re using flocculants that produce much smaller bubbles which lock up the air a lot more than brittle froths.
“We’re working together with customers to find ways to manage these more tenacious froths, by looking at their circuit design and dealing with areas where the air could accumulate and block the system, paying particular attention to their pumps, pipes and sumps,” he said.